1. Field of the Invention
The subject matter of this invention relates generally to automatic transfer switches (ATS) and more particularly to microprocessor-controlled automatic transfer switches.
2. Description of the Prior Art
Automatic transfer switch devices are described, for example, in U.S. Pat. No. 3,936,782 issued Feb. 3, 1976 to Moakler et al. and entitled "Automatic Transfer Switch" and U.S. Pat. No. 4,189,649 issued Feb. 19, 1980 to Przywozny et al. entitled "Control Panel For Automatic Transfer Switch". Examples of automatic transfer switches and control devices associated therewith are also described in the following brochure:
"WESTINGHOUSE TRANSFER SWITCHES," SA-10915 by the Westinghouse Electric Corporation, Low Voltage Breaker Division.
"ENGINEERING DATA AUTOMATIC TRANSFER SWITCH," Bulletin ATS-100A, Russelectric, Inc., May, 1984
"ZENITH ZTS TRANSFER SWITCHES," Bulletin 0-5021 (REV. 2), Zenith Controls, Inc.
"AUTOMATIC TRANSFER SWITCHES," Bulletin SP-44, Square D Company.
"POW-R-TRAN.RTM. SOLID STATE AUTOMATIC TRANSFER CONTROLLER", Catalog Section 31-550, Dec. 6, 1976, Westinghouse Electric Corporation, Distribution Equipment Division.
Automatic transfer switches are devices that switch a power source for a load from a primary to a secondary source automatically or after manual operation for any number of important reasons. Automatic transfer systems are often found in hospitals, subways, schools, airports, office buildings and other commercial structures equipped with secondary power sources. Basically, automatic transfer systems come in either of two different types. One utilizes a double-throw contactor device while the other type utilizes circuit breakers as the primary switching devices. Furthermore, some kinds of automatic transfer switches systems utilize relay logic exclusively in the control portion thereof. Other types of automatic transfer switch systems utilize solid-state circuit devices to replace the relay devices. In addition, there are automatic transfer controllers which can be used in conjunction with separate automatically operated circuit switching devices such as circuit breakers, or fused or unfused switches. The controller supplies the intelligence to these primary switching devices when automatic transfer of load from one source to another becomes necessary. None of the known automatic transfer switch systems use microprocessor control. It would be advantageous to find an automatic transfer switch or switch system which utilized microprocessor control.
It is known to utilize microprocessor control in other types of circuit systems or devices such as motor control apparatus. An example, of such a device is described in U.S. Pat. No. 4,453,117 issued June 5, 1984 to R. T. Elms et al. and entitled "Motor Control Apparatus with Short Term Undervoltage Motor Mode Saver".
It would be advantageous to have a simple microprocessor-controlled automatic transfer switch controller which could be universally utilized on either the contactor type of system or the circuit breaker type of system.
Most automatic transfer switch systems have as a part thereof a mechanical clock which is utilized by the automatic transfer switch system to delay switchover. It is known that automatic transfer switches are often used to transfer electrical power from a normal source of power to a generator driven emergency source of power. The clock may be utilized to prevent spurious starts of the generator system. For example, normal power may be periodically lost for a period of one or two seconds and then be reestablished. In some applications it is a standard practice for an occurrence such as this not to cause a start the emergency system. If, however, the power remains off for a longer period of time, it is usually necessary to start the emergency system. It would be advantageous if a clock could be provided which was non-mechanical in nature and which was controlled by a microprocessor and was preprogrammable over a range of delay time to prevent starting of the emergency system until the programmed time delay has taken place. This would guarantee that the starting of the system was desired rather than the result of a spurious short-term absence of normal power.